This invention relates in general to the clamping of workpieces to a surface and, more specifically, to a system for indexing and clamping a workpiece to a base plate that leaves the opposite surface free for machining or the like.
When raw material or preforms are to be machined it is necessary that the workpiece be tightly clamped to a base plate or an intermediate sub-plate to hold the workpiece stable at a precise position while resisting the forces on the workpiece produced by the machining operation. Typically, such machining includes drilling, boring, honing, grinding and milling.
In numerical controlled machining, a machine tool, such as a milling head, is programmed to follow a very precise path. The workpiece must be located extremely precisely relative to certain machine datum points and/or datum planes from which the machining path is related or measured.
A number of different indexing and clamping systems have been developed to permit a workpiece to be positioned at a specific location on the support plate. Generally, the clamps have fingers and straps or other means that extend over the side of the workpiece opposite the supporting surface. While generally effective, these upper surface clamps must be carefully positioned to avoid interference with the movement of the machine tool, such as a milling head, across the workpiece.
Insufficient clamping pressure, or the use of too few clamps, may allow the workpiece to shift during machining, resulting in wasted, out-of-tolerance products. Further, errors in clamp placement may allow the machine tool cutter to run against the tool, resulting in damaged clamps and serious damage to the machining tool and/or to the machine itself and could be a safety hazard to nearby personnel.
A wide, generally flat workpiece that is clamped to a support plate around the edges may lift slightly at the center due to machining forces. Thus, the central dimensions of the product may be thinner and out-of-tolerance.
It is absolutely essential, especially with numerical controlled machining, that the workpiece be positioned precisely relative to datum points or planes on the support plate. Failure to establish and maintain the precise workpiece position will result in an improperly machined and rejected part.
A variety of components have been used to accurately locate workpieces. Among these are index pins mounted on or in the support plate and extending into precisely located holes in the workpiece. Round pins inserted into round workpiece holes may have problems such as lack of the necessary concentricity, a hole that has an axis that is slightly off the pin axis (angularly or otherwise), pins that tend to interfere or be loose in productions holes with variable diameters, etc. These problems, and others, largely keep full round index pins from having the necessary overall accuracy and flexibility.
Thus, there is a continuing need for improved indexing and clamping systems for workpieces to be machined, systems that have improved reliability, accuracy and versatility and do not extend over or obstruct the workpiece surface being machined, generally the surface opposite the surface in contact with a support or base plate.